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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Expanding of the bandwidth of slow light by artificial inhomogeneous broadenings

De-Kui Qing, Zhijie Deng, Philip Hemmer, Marlan O. Scully, Muhammad Suhail Zubairy

Published Feb 9, 2006• 0 citations
Publisher

Abstract

Recently artificial inhomogeneous broadening was proposed to expand the bandwidth of slow light. The point is to independently slow down all harmonic components of the input pulse via inhomogeneous broadening. An input pulse or sequence of pulses can be split into independent spectral channels by a dispersive element such as a prism or grating. These sub-pulses are then slowed by bandwidth-matched slow-light array elements, and then recombined with another dispersive element to produce the output pulse. The proof of principle experiment was done with a photorefractive crystal Ce:BaTiO3 where the crystal function as both dispersive elements and slow lights devices.

DOI
10.1117/12.660171

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Recently artificial inhomogeneous broadening was proposed to expand the bandwidth of slow light. The point is to independently slow down all harmonic components of the input pulse via inhomogeneous broadening. An input pulse or sequence of pulses can be split into independent spectral channels by a dispersive element such as a prism or grating. These sub-pulses are then slowed by bandwidth-matched slow-light array elements, and then recombined with another dispersive element to produce the output pulse. The proof of principle experiment was done with a photorefractive crystal Ce:BaTiO3 where the crystal function as both dispersive elements and slow lights devices.
For many applications of slow or stopped light, the delay-time-bandwidth product is a fundamental issue. However, existing slow light demonstrations do not give a satisfactory delay-time bandwidth product, especially in room temperature solids. Here we demonstrate that the use of artificial inhomogeneous broadening has the potential to solve this problem by simultaneously slow down all the frequency components of the input pulse. The proof of principle experiment was done using three-wave mixing in a photorefractive crystal Ce:BaTiO3 where Bragg selection is used to provide the inhomogeneity.
For many applications of slow or stopped light, the delay-time-bandwidth product is a fundamental issue. So far, however, slow-light demonstrations do not show a large delay-time-bandwidth product, especially in room temperature solids. Here we demonstrate that the use of artificial inhomogeneous broadening has the potential to solve this problem. A proof-of-principle experiment is done using slow light produced by two-beam coupling in a photorefractive crystal Ce:BaTiO3 where Bragg selection is used to provide the artificial inhomogeneity. Examples of how to generalize this concept for use with other room temperature slow-light solids are also given.
Recently, several works have shown that the light group velocity can be reduced using the recording of dynamic refractive index gratings in photorefractive (PR) crystals [1] , [2] , [3] . The slowdown of light is performed using optical beams wave mixing: it yields the output pulse amplification and increases the light dispersion inside the PR crystal. For example, the TWM method demonstrated that the delay time ∆ τ can be varied as a function of the TWM gain values Γ and the input pulse widths T 0 . However, several works have shown that the results achieved by this method are limited in terms of pulse duration. Indeed, only long pulses with time duration of ms−s can be slowdown in a PR crystal both at the visible and the infrared wavelengths [4] .
The dispersion experienced by a signal in a slow light system leads to a significant pulse broadening and sets a limit to the maximum delay actually achievable by the system. To overcome this limitation, a substantial research effort is currently being carried out, and successful strategies to reduce distortion in linear slow light systems have already been demonstrated. Recent theoretical and experimental works have even claimed the achievement of zero-broadening of pulses in these systems. In this work we obtain some physical limits to broadening compensation in linear slow light systems based on simple Fourier analysis. We show that gain and dispersion broadening can never compensate in such a system. Additionally, it is simply proven that all the linear slow light systems that introduce a low-pass filtering of the signal (a reduction in the signal root-mean- square spectral width), will always cause pulse broadening. These demonstrations are done using a rigorous shape-independent definition of pulse width (the root-mean-square temporal width) and arguments borrowed from time-frequency analysis.
We describe a procedure for increasing the fractional delay (or delay-bandwidth product) of a slow-light system. A broadband input signal is sliced into several frequency bands. The light in each band propagates through a separate channel which possesses a highly reduced group velocity over a narrow frequency band. The output of each channel is then combined to form a single output field. For certain discretely distributed values of the group velocity of each channel, the output can replicate the input waveform without the need to adjust the output phase of each channel. Because this scheme makes use of many parallel channels, it can overcome a fundamental limit [D. A. B. Miller, Phys. Rev. Lett. 99, 203903 (2007)] to the delay-bandwidth product for single-channel devices. A practical design is proposed using spectral slicers and stimulated Brillouin scattering as the narrow-band slow-light process. Numerical simulation shows that such a channelized slow-light element can have nearly uniform gain and group index over very large bandwidths.